Harry Potter grew up over the course of Joanne Rowling's seven books.
At first it was addressed at ten-year-olds, but as it became a stellar publishing phenomenon, the Harry Potter world became more serious. That is, the writing focused more on plot and less on the twee hokum that was meant to draw in the ten-year-olds, and which strongly characterised the first book.
It wasn't as if Rowling did a complete volte face to address a different and larger - adult - audience. A little of the hokum remained. But by the end of the last book, it is obvious the whole series was mapped out in advance to a stunning level of detail. But Rowling's content changed to accentuate what was compelling in the books - the intricacy of the plotting, the fervid imagination, the consistency of detail, the complexity of characters, and above all, a nuanced but strong ethical framework. The level at which these are achieved are not the hallmark of a merely competent writer caught up in a maelstrom of unexpected success (such as Dan Brown).
Once the final book is read, pick up a book at random from the middle of the series. You will find many details that are offloaded in passing, which don't seem to have specific plot relevance. Those same details then come with a much more meaningful context. ('"No - [spying on the deatheaters is] your job, isn't it"..."Yes, Potter" [Snape] said, his eyes glinting"' ). Of course, there are a lot of red herrings, but in a different light it's plain to see how some ideas have deeper meaning, while others simply mirror the myriad other possibilities that don't bear out.
There was an article doing the international rounds, a column from a British journalist: "Why I hate Harry Potter". But the writer was simply reacting to the Phenomenon - obviously hadn't read enough of the books to realise what a treat she was in for, if she'd had more wit and patience. That journalist shall be nameless, not like Rowling.
These books will have lasting significance, long after the films are relegated to archival status.